Body Structure and Symmetry

Mollusks have a very distinct body structure of a significant cavity used for breathing and excretion. Mollusks have a body without a cavity, has more than two layers of cell layers, tissues and organs. Mollusks are soft bodied animals with three main regions: head, visceral mass, and foot.

The head of the mollusk contains the sense organs such as the brain. It has a pair of eyes, a mouth, sensory and nervous structures. It also has a radula, their teeth.

VISCERAL MASS The visceral mass is the fleshy, boneless, body of the mollusk which contains all the internals of mollusks such as digestive system, excretory system and circulatory system. The tissues of the organs are covered by a mantle. It is a skin-like organ that holds the materials in the shell.

This is the muscular lower part of the body, mainly used to help move themselves around, but each foot for each mollusks have its own purpose. Their foot is usually in contact with a substance. The foot carries a pair of statocysts, which act as balance sensors. In gastropods, it secretes mucus for movement. In species that only have one shell, their foots act as a sucker attaching themselves to a hard surface, clamping the shell down over it. In two shelled mollusks, their foot is designed to burrow beneath the earth and cephalopods' foot is modified to tentacles and arms for jet propulsion.

In some mollusks, they have a shell. The shells is made of calcium carbonate and consists of chitin and conchiolin. The shell has three layers: outer layer made from organic matter, middle layer made of columnar calcite and inner layer of laminated calcite. For most mollusk, their shell is external including snails, scallops, oysters, clams and mussels. For other mollusk, like squid, and cuttlefish, they have a internal skeleton similarly to a shell. There are also some that do not have a shell at all, like octopus and slugs, with only a mantle for protection of organs.

Mollusks have bilaterally symmetry, meaning that they have a body arrangement that is symmetrical, along the center. Also, when the body is divided into two, they are alike.